The origin of the avian cardiovascular system from the primitive streak has been mapped by the construction of quail/chick transplantation chimeras, the use of QH-1 (an antiquail endothelial/endocardial cell marker), and injections of a vital fluorescent dye (DiI) into the primitive streak. Our studies reveal that the prospective heart, including its endocardial and myocardial layers and the adjacent parietal pericardium, occupies much of the rostral half of the primitive streak at early primitive-streak stages of gastrulation. The heart originates from the primitive streak in roughly rostrocaudal sequence (i.e., the prospective bulbus cordis arises more rostrally in the streak than does the prospective ventricle, which in turn arises more rostrally than does the prospective sinus venosus), and all layers of the heart at each of its rostrocaudal subdivisions originate in concert from the same level of the primitive streak. Moreover, all rostrocaudal levels of the primitive streak at gastrula and neurula stages contain prospective endothelial cells. Again, these cells are rostrocaudally ordered within the streak, such that head blood vessels (both arteries and veins) arise at more rostral streak levels than do trunk blood vessels. Ingression of cardiogenic cells is complete by midprimitive-streak stages, and the position formerly occupied by prospective cardiogenic cells within the streak becomes occupied by ingressing prospective somitic cells. Additional studies on the state of commitment of prospective cardiogenic and endothelial cells during their ingression through the primitive streak are underway.